Adapting Faroe Current monitoring to a reprocessed altimetry data set

The monitoring system for the Faroe Current uses available in situ observations of temperature and salinity, but also relies heavily on satellite data from eight altimetry grid points (A1 to A8). Using algorithms documented in a number of publications (H2015, H2019, and H2020), time series of the four monitoring parameters (volume transport of Atlantic water, heat transport relative to 0°C, and transport-averaged temperature and salinity) have been updated whenever the altimetry data set was updated. With the altimetry data update in December 2021, it was clear, however, that there were major changes in the data set; not only in the latest part, but throughout the altimetry period, beginning in 1993. This raised two main questions:

- Are the relationships, on which the algorithms are based, still valid?
- Which equations and coefficients need to be modified?

Answering these questions was the main aim of this report. The equations and algorithms have been developed by comparing altimetry data with various types of in situ observations. To answer the two questions above, we have therefore re-done the analyses, mostly in the form of regression analyses. The result was unequivocal. All the relationships were still valid and for all of them, the explanatory power increased with the new altimetry data set, although some of the coefficients had to be modified.

In this process, a number of results and conclusions of more general interest have emerged. These have been published in a scientific manuscript (Hansen et al., 2023) and the interested reader should refer to that publication. This report duplicates some of these results in more detail, but is mainly intended to provide a more detailed documentation of the monitoring system for future reference.

In spite of higher explanatory power and some coefficient modifications, the time series generated from the new altimetry data (e.g., volume transport) only differ slightly from the series generated from the old data. The new analyses also support our previous conclusion that in situ current measurements are no longer necessary to monitor the velocity structure of the Faroe Current, although we recommend that one of the long-term ADCP mooring sites is maintained to guard against potential drastic changes in the system.